Storage tube screens



July 30, 1968 D. D. DUGGAN STORAGE TUBE SCREENS 2 Sheets-Sheet 1 FiledDec. 14, 1964 1 I I l I 1 I INVENTOR.

DANIEL D. DUGGAIV ATTORNEY July 30, 1968 D. D. DUGGAN 3,395,304

STORAGE TUBE SCREENS Filed Dec. 14, 1964 2 Sheets-Sheet 2 INVENTOR.

DAN/EL 0. 0066A aw/aw ATTDRNEY 3,395,304 STORAGE TUBE SCREENS Daniel D.Duggan, Roanoke, Va., assignor to International Telephone and TelegraphCorporation, a corporation of Delaware Filed Dec. 14, 1964, Ser. No.418,251 11 Claims. (Cl. 313-89) ABSTRACT OF THE DISCLOSURE An electronbeam image storage tube has a gradient transmission screen wherein avarying dimension thereof provides a greater impedance to the passage ofelectrons through the center portions than at the edges to achieve arelatively uniform erasure of a stored charge.

This invention relates to storage tube screens and particularly to agradient transmission screen arrangement and method which providesvarying electron transmission through different areas of the screensurface to compensate for differences in erasure time of a stored chargecaused by non-uniform current density of the flood beam.

The usual electron beam storage tube of the direct viewing type containswriting and flood guns at one end and storage and display screens at theother end. The electron beam from the writing gun is intensity modulatedby a signal and scanned across the storage screen, which is formed of afine mesh metal grid and an insulator layer having secondary emissionproperties, to selectively charge the screen in accordance with thesignal. An adjacent collector screen collects the secondary electronsemitted from the storage screen. A flood gun in one mode applies anexpanded beam of electrons over the entire storage screen which controlsthe passage of the beam therethrough to display the charge pattern onthe phosphor coated face of the tube. In the case of an electricalreadout tube, an electrical output signal representing the stored imageis derived from a dielectric coated metal backing plate of the screen.In a second mode, the flood beam is permitted to strike the storagescreen to erase the charge. Due to inherent non-uniformities in theflood beam, such as the greater concentration or density of electrons atthe center, which cannot be perfectly corrected by collimating or focuselectrodes, and the variation of the radial path at which the electronsstrike the different surface areas of the planar storage screen, theerasure time is not the same at all portions of the screen. The edgeareas of the screen thus generally erase before the center, since thecenter electrons tend to pass through more readily than the outerelectrons which are intercepted. In addition, a higher screen voltage isrequired to cut-off the more dense beam portions.

In some cases, the voltage setting may permit only the edges of thescreen to erase while the center maintains the charge. A partialsolution to the problem of providing a constant radial path length wasthe use of a curved screen, such as described in US. Patent No.3,201,628, issued Aug. 16, 1965, and assigned to the same assignee asthe instant application. This form, however, introduced structuralcomplexities and did not compensate for the non-uniform beam currentdensity.

It is therefore the object of the present invention to provide a simplereliable structure and method to compensate for variations in erasuretime of a storage screen.

It is another object to provide a gradient electron transmission anduniform erasure for a storage surface while maintaining high writingspeed brightness.

These objects are achieved by a novel arrangement which changes theelectron transmission properties of the States Patent Patented July 30,1968 mesh screen by using a graded material such as an insulator layerwhich varies in dimension from the center to the edges of the screen.Thus, thicker insulator layers which impede the electrons more andtherefore erase faster are placed at the center to compensate for thenormal lag in that area. Another uniform high secondary emissioninsulator coating may be applied to maintain a .high writing speedbrightness. A novel mask and evaporation process provide the desiredgraded layer structure.

The details of the invention will be more fully understood and otherobjects and advantages will become apparent in the following descriptionand accompanying drawings, wherein:

FIG. 1 shows a top view of the apparatus, mask and fragmented screensection used to form a graded thickness insulator coating;

FIG. 2 shows a partial side view and cross section of the sameapparatus; and

FIG. 3 is representative fragmentary enlarged cross section of theinsulator screen showing the details of the storage screen layers.

As shown in FIGS. 1 and 2, an insulator storage screen 10 is positionedin an evacuated bell jar 12 and secured on an insulated mounting ring 14and a geared rotary support 16. The mechanism is driven by an externalvariable speed motor 18 having a magnetic coupling 20 and an internalfollower 22 with a driver gear 24. Two idlers 26 provide additionalsupport. The mounting ring is preferably flat to reduce edge effectsduring the evaporation process. The storage screen is rotated over anevaporation boat 28 containing a particular coating material. The boatis connected between a pair of electrodes to heat and evaporate thematerial in a known manner. A special mask 30 is positioned between theboat and screen to cause the evaporated insulator material to bedeposited on the screen as a graded layer with the center of the screenhaving a greater thickness than the periphery or outer edges. This isachieved by the particular curve shape of the mask wherein a greaterportion of the screen is blocked out at the edges than at the center tocompensate for the inherent non-linearity of the flood beam electrons,with rotation of the screen providing a symmetrical coating distributionabout the central axis. Other suitable masking devices such as aphotographic type shutter or iris may similarly be employed.

The resultant structure is shown in an exaggerated form in FIG. 3, withcoating 32 of magnesium fluoride, for example, forming the gradedinsulator layer on metal grid 34. Other suitable high refractorymaterials such as calcium or barium fluorides may similarly be employed.The meal grid is preferably formed of a suitable metal such as nickelwhich may be pre-aluminized to seal and protect the metal from reactionwith the other coatings. The storage screen may then be furtherprocessed by providing a metal coating 36, such as gold or aluminum, onthe backside to cover any stray insulation material which may have beenevaporated behind the screen. An additional coating 38 of magnesiummonoxide or other suitable high secondary emission material, such assilicon monoxide, may then be added with a uniform thickness to reducethe effect of the increased center thickness on the writing speedbrightness. This is the speed at which the writing beam can charge theinsulator surface to establish a given brightness on a display screen.This latter coating has a sutficiently high secondary emission ratio sothat the writing speed increases to the extent that any increase causedby the thicker center layer is negligible. Thus the magnesium fluoridelayer controls the erase time While the magnesium monoxide layercontrols the writing speed brightness.

In a typical arrangement, seven fringes or incremental portions of alayer of magnesium fluoride, as measured by fringe counter 40, areevaporated onto the nickel screen, each fringe being about 0.15 microns,with a gradation of approximately 1 to 3 fringes existing between thecenter and outer periphery, depending upon the flood beam currentcharacteristics. A fast binding layer is evaporated onto the screenfirst, with slower evaporation of the bulk of insulator materialfollowing. The gold layer on the opposite side may be about one fringewith the magnesium monoxide layer having a three fringe thickness. Adetailed description of a fringe counter is found in an article entitledControl of the Thickness of Evaporated Layers During Evaporation by G.Papp, published in the Review of Scientific Instruments, October 1959,vol. 30, pp. 911-912. As a result of this arrangement, whereas prior artscreens have been found to have erasure time variations in the order ofbetween 0.8 to 3.0 second from the edges to the center, the presentinvention provides a substantially uniform time of about 1.5 seconds.

In addition to the graded insulating material, other variations of theinstant invention which have similar effects in producing uniformerasure time, may include the provision of a graded metal backing layer,the use of a variable apertured metal grid having closer spacedapertures 0r thicker metal coatings creating smaller apertures at thecenter than at the edges, or a variable coating or graded mesh on thecollector screen adjacent the storage screen. It may thus be seen thatthe present invention provides a novel screen structure and coatingmethod which changes the electron transmission characteristics of thestorage elements to compensate for variations in current density of theflood beam and achieves uniform erasure at all areas of the storagescreen. While only a single embodiment has been illustrated, it isapparent that the invention is not limited to the exact form or useshown and that many other variations may be made in the particulardesign and configuration without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:

1. A screen for an electron beam type storage tube including means forstoring and erasing a charge image comprising a fine mesh metal grid andgradient transmission means thereon having a varying dimension providinggreater impedance to the passage of electrons through the centerportions of said grid than at the edges sufiic-icnt to provide arelatively uniform erasure of said charge.

2. The device of claim 1 wherein said gradient transmission meanscomprises a first layer of insulating ma terial on one side of saidgrid, said insulator layer having a greater thickness at the centerportions than at the edges.

3. The device of claim 2 including a second layer of insulating materialon said first layer havinga uniform thickness and high secondaryemission characteristics.

4. The device of claim 2 wherein said first insulator layer is of a highrefractory material.

5. The device of claim 2 wherein said grid is formed of nickel andincludes an alumnum coating sealing the nickel surface.

6. The device of claim 2 wherein said insulator thickness is between0.10 and 0.50 micron greater at the center than at the edges.

7. The device of claim 3 including a uniform thickness metallic layer onthe opposite side of said grid.

8. The device of claim 4 wherein said first layer is formed of amaterial selected from the group consisting of magnesium fluoride,calcium fluoride and barium fluoride.

9. The device of claim 4 wherein said second layer is formed of amaterial selected from the group consisting of magnesium monoxide andsilicon monoxide.

10. The device of claim 7 wherein said metallic layer is formed of amaterial selected from the group consisting of gold and aluminum.

11. The device of claim 8 wherein said second layer is formed ofmagnesium monoxide.

References Cited UNITED STATES PATENTS 2,979,633 4/1961 Harris 3 l3893,089,050 5/1963 Lehrer 3l3-68 3,242,367 3/1966 Slegho 313-89 FOREIGNPATENTS 612,033 1/1961 Canada.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

